The refinement of buffeting analysis for long span bridges in the frequency domain based on parameters obtained from field measurement systems and wind tunnel experiments is the object of this thesis. As part of the aerodynamic investigation for the unprecedented 1018m span Stonecutters Bridge, a field measurement system for studying wind turbulence characteristics was installed at the bridge site. The system includes the installation of ultrasonic and mechanical anemometers, bi-axial accelerometers, etc. on a 50m high meteorological mast near the eastern pylon of the bridge. In order to work out the correlation of turbulence characteristics between the mast and the bridge line, 1:1500 scale terrain model wind tunnel experiments for the study of the flow field in the vicinity of the bridge site was conducted. Furthermore, a series of 1:80 scale sectional model experiments have been conducted to evaluate the steady state aerodynamic force coefficients, flutter derivatives and spatial distribution of the aerodynamic forces on deck section of Stonecutters Bridge, including the chord-wise aerodynamic admittance functions and span-wise root coherence functions of forces. Through the wind characteristics studies, a mathematical turbulence model suitable for the buffeting calculations was derived. This will serve as a good reference for other projects in the area and a firm background for further development of buffeting analysis methodology. The sectional model experiments have reinforced and enhanced the understanding of the aerodynamic characteristics of twin-girder deck sections. The effect of gap width on flutter derivatives, aerodynamic forces, chord-wise aerodynamic admittances and span-wise root coherence of forces has been looked at in detail. The effect of Reynolds number (Re) on the evaluation of steady state aerodynamic forces coefficients has also been studied in detail. The findings will be a useful reference for future projects employing a similar separated twin-girder deck section.Stonecutters Bridge has adopted a special separated twin-box aerodynamic deck section with a wide gap of 14.3m between the twin girders. This kind of deck section in real-life application is extremely limited. In order to test the sensitivity of this kind of deck section to aerodynamic forces, the pressure measurement experiments were organized in a parametric study that aimed at defining the influence of width of gap in relation to the scales of turbulence on the dynamic loading. Empirical models for the gust loading on the twin-box girder deck sections were proposed. In lifting the traditional approach based on wind coherences, a new buffeting response analysis method based on the directly measured root coherence functions of forces and aerodynamic admittance function is proposed. The analytical results obtained using the new methodology were found to match reasonably well with those obtained in the full bridge aeroelastic model experiment. The new method is therefore considered reliable